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It has been a while since I read an actual book so at the beginning of this year, I set a goal: 12 books this year (as of now, I’ve read six). The first of these books was Frostbite by Nicola Twilley. It’s a book about how advances in refrigeration changed our relationship with food. Early on in the book, when Twilley described her experience visiting cold-storage facilities, I came upon a page that talks about why we catch more colds in the winter. The book mentioned that at 4°C, our noses release fewer “defensive bubbles” that protect against viruses than they would otherwise in warmer temperatures.
As a microbiologist, I had to look up the paper.
This paper was published two years ago in the Journal of Allergy and Clinical Immunology by researchers based in Boston and Cairo.
Studies of how environmental conditions affect viral infections
FIrst some background. We commonly think that respiratory illnesses increase in the winter because we’re stuck indoors with one another, breathing in the same air. But the paper actually describes many other studies on how environmental changes affect viral illnesses. Some observations from other studies:
- Some viruses replicate better at lower airway temperatures
- Temperature and/or humidity can affect how likely we’re going to get a virus
- Cold temperatures can dampen our immune responses to viral infections
Defending the respiratory tract from pathogens
Our nasal passages are our first point of contact with pathogens, but fortunately, the body is prepared with several defenses against inhaled pathogens. Mucus. Antimicrobial peptides. And, receptors on the cells of our nasal passages, which recognize patterns commonly found on pathogens, trigger an immune response.
In 2018, these researchers published another paper that found another line of defense in our nose: extracellular vesicles that get released from cells in our nasal passages and can bind microbes to block them from entering our cells. The researcher’s 2018 study saw that the vesicles acted on bacteria and their more recent study in 2022 found they acted on viruses too. The vesicles also carried precious antiviral cargo such as miRNA molecules that can block viral replication.
In their work, the team determined a sequence of events that triggered vesicle release:
- Receptors on our cell surface recognize the microbe and binds it
- This triggers the vesicle release
- Vesicles can (a) bind and sequester the microbes, preventing them from binding nasal cells and they can (b) be taken up by surrounding cells, transferring antiviral cargo to them
(this graphic summarizes this)
Lowered temperatures weaken this response
To begin studying the effects of cold weather on these vesicles, the researchers first had to know how much colder the nose gets in cold weather. They found that the nasal temperature drops about 5°C. The researchers took this temperature change into the lab and tested it on nasal cell cultures at 32°C (vs. normal body temperature of 37°C).
To sum up their work, they found that at the colder temperatures, there was a 42% decrease in the number of vesicles released, and that of the vesicles released, they produced fever antiviral miRNA molecules and bound fewer viruses.
The researchers propose that in the future, it’s possible that new therapeutics that boost extracellular vesicle secretion, could help improve the immune system in the nose.
This research, although super cool, leaves me with a few questions. I’m curious if simply covering your nose outside (and hopefully keeping it a bit warmer) can help bring up temperatures so we don’t see the drop in vesicle numbers and effectiveness. I’m also wondering if this effect depends on the amount of time we spend outdoors. The study had people spend 15 minutes at 4.4°C before taking their nasal temperature with an endoscope before testing cell cultures at that same temperature. However, they grew these cell cultures at 32°C for 24 hours so would that mean for us to see similar decreases in vesicle function in our noses, we would need to be in the cold for 24 hours? Brrrr!
Featured image: https://pxhere.com/en/photo/863629
